If really embedded, you may not be able to polish out.  But for more sufferficial surface build-up, I’ve had reasonably good results with common 409 cleaning spray.  Spray on, let sit for 5 minutes.  Repeat if you seem to be making head-way. This issue made us totally give up on ZYP and boron in general.  Life has been much better (for me & spouse) with better kiln wash and kiln wash technique. The haze left by boron is unacceptable on most pieces.

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